The invention relates to a data transmission facility between two asynchronously controlled data processing systems. The facility has a buffer memory constructed of a plurality of memory sections for the intermediate storage of a respective data word, and also two control devices. One control device is an input control and controls the transfer of one respective data word into a memory section of the buffer memory available for the transfer synchronously with a system clock of the system emitting the data. The other control device is an output control and controls the forwarding of one respective data word from the memory section of the buffer memory available for output to the system accepting the data, and controls the forwarding synchronously with the system clock of the system accepting the data.
Corresponding data transmission facilities are known, for example, from the IBM Technical Disclosure Bulletin, Vol. 10, No. 1, June 1967, pages 34-36, incorporated herein by reference. With this known arrangement, there is the disadvantage that data words, under certain conditions, can be lost during the transmission between the two asynchronously functioning systems when a release or turnoff of the buffer memory overlaps with an input or, on the other hand, when a call-signal overlaps with a load in the buffer memory such that the corresponding circuit elements are situated in a status in the time span critical for the transmission which is not unambiguously logically defined or metastable, as is explained, for example, in "Frequenz" 31 (1977) 3, Pages 71 through 76, incorporated herein by reference.
The arrangement known from U.S. Pat. No. 4,070,630, incorporated herein by reference, does not function satisfactorily in this regard even though the call-in by the free buffer memory is correlated with the trailing edge of the individual clock pulses of the system clock of the outputting data process system.